The present invention relates to deflection yokes for deflecting the electron beam to be emitted by a cathode-ray tube.
With reference to FIGS. 10(a) and 10(b), two annular centering magnets 7, 7 are mounted on a deflection yoke 6 at its neck end. The centering of an electron beam can be adjusted by rotatingly moving the magnets 7, 7.
The centering magnet 7 is molded in an annular form as shown in FIG. 11 from a resin containing a finely divided magnetic material mixed therewith (hereafter referred to as the xe2x80x9cmagnetic resinxe2x80x9d). The magnet is magnetized to an N pole 73 and an S pole 74 at two portions which are displaced from each other by 180 degrees about the center axis of the annular form to serve as a two-pole magnet. The centering magnet 7 has a pair of adjusting knobs 70, 71 projecting from the two portions where the N pole 73 and the S pole 74 are provided. The annular form of the centering magnet 7 is symmetrical about an extension (line Exe2x80x94E) of a diametrical line connecting the center point 75 of the N pole 73 to the center point 76 of the S pole 74, and is also symmetrical about an extension (line Fxe2x80x94F) of a diametrical line orthogonal to the line Exe2x80x94E.
Examples of useful finely divided magnetic materials are those of the alnico (Alxe2x80x94Nixe2x80x94Co) type, ferrite type or rare-earth elements. Finely divided alnico magnetic materials are widely used from the viewpoint of magnetic intensity, temperature characteristics, cost and productivity.
However, the conventional centering magnet 7 prepared from a magnetic resin containing a finely divided alnico magnetic material mixed therewith has the problem that the images displayed on a picture tube of electron beam are low in resolution because of an uneven distribution of the finely divided magnetic material. FIG. 12 shows the distribution of finely divided magnetic material in the centering magnet 7. The content of the magnetic material 31 is greater toward the N pole 73 and decreases toward the S pole 74 presumably because the finely divided alnico magnetic material is greater than ferrite magnetic materials in particle size.
In the molding step wherein the magnetic resin is used, the magnetic resin is poured into a mold 8 having an annular hollow portion 80 through a resin gate opening 81 thereof as shown in FIG. 13 and then divided into two resin streams along the annular form of the hollow portion 80. The divided streams thereafter flow together. Consequently, the content of the finely divided magnetic material is small toward the resin gate opening 81 and increases at a confluent portion 79 shown in FIG. 12 where the two resin streams meet, with the result that uneven magnetization occurs in the centering magnet 7 having the N pole 73 and the S pole 74 formed by magnetization, between the S pole (74) side having a small magnetic material content and the N pole (73) side with a great magnetic material content.
FIG. 14 shows a distribution of lines of magnetic force produced across the central aperture 78 of each of centering magnets 7, 7 which are arranged in a pair as positioned in opposite relationship in polarity. As illustrated, lines of magnetic force emanating from the N pole 73 toward the S pole 74 are arranged concentrically on the N pole (73) side of high magnetic material content and dispersed on the S pole (74) side of low magnetic material content, thus spreading out from the N pole 73 toward the S pole 74. Accordingly, the point of change of polarity from the N pole 73 to the S pole 74 on the centering magnet 7 is positioned closer to the N pole 73 than the line Fxe2x80x94F, resulting in a difference between the distance from the N pole 73 to the point of polarity change and the distance from the S pole 74 to the point. When the two centering magnets 7, 7 are superposed on each other as positioned in opposite relationship in polarity, the lines of magnetic force emanating from one of the magnets 7, 7 will consequently intersect those from the other magnet as seen in FIG. 15.
FIG. 15 shows that the two centering magnets 7, 7 are positioned at an angle of zero relative to each other. When the relative angle is altered from this position, the lines of magnetic forces of the two magnets 7, 7 combine, and the combined lines of magnetic force exert on the electron beam a magnetic force for the adjustment of centering (adjusting magnetic force). Even if the two centering magnets 7, 7 are set in the position of relative angle of zero as shown in FIG. 15 in the case where no adjustment is needed for centering, the distribution of the lines of magnetic forces of the magnets 7, 7 is not symmetrical about the line Fxe2x80x94F, and the magnets 7, 7 therefore fail to offset each other in magnetic lines. As a result, the combined magnetic lines set up a four-pole residual magnetic field over the central apertures 78, 78 of the two magnets 7, 7 as indicated by allows in FIG. 16.
Accordingly, the electron beam 9 having a circular cross section and passing through the central apertures 78, 78 of the magnets 7, 7 is expanded in one direction and contracted in other direction orthogonal to the direction by being magnetically acted on by the four-pole residual magnetic field to deform to an elliptical form in cross section. This impairs the performance of image focusing on the screen of the picture tube of electron beam to form images of low resolution.
An object of the present invention is to provide a centering magnet adapted to eliminate the adjusting magnetic force when there is no need for centering adjustment, a deflection yoke provided with such centering magnets, and a process for producing the centering magnet.
The present invention provides a first centering magnet which is molded in an annular form from a magnetic resin, the centering magnet having two portions opposed to each other as displaced from each other by 180 degrees about a center axis of the annular form thereof and magnetized to an N pole and an S pole respectively, the content of finely divided magnetic material varying circumferentially of the annular form, the distribution of the content being symmetrical about a straight line (line Bxe2x80x94B) orthogonal to a straight line (line Axe2x80x94A) through a center point of the N pole and a center point of the S pole.
The invention provides a first deflection yoke having two first centering magnets mounted thereon in combination.
The distribution of the content of the finely divided magnetic material in the first centering magnet of the invention is symmetrical about the line Bxe2x80x94B, so that the distribution of the lines of magnetic force emanating from the N pole toward the S pole is similarly symmetrical about the line Bxe2x80x94B.
With the first deflection yoke of the invention, the distribution of the lines of magnetic force emanating from the N pole toward the S pole in each centering magnet is symmetrical about the line Bxe2x80x94B, so that when the two centering magnets are superposed on each other as positioned in opposite relationship in polarity, the lines of magnetic forces emanating from one of the two magnets offset those of the other magnet, setting up no four-pole residual magnetic field due to the combination of magnetic lines. This eliminates the magnetic force for the adjustment of centering.
The present invention further provides a second centering magnet molded in an annular form from a magnetic resin, with a plurality of positions arranged symmetrically about a diametrical line of the annular form and each serving as the most upstream position for a flow of the resin for molding, the centering magnet being magnetized to an N pole and an S pole respectively at two portions intersecting the diametrical line or at two portions 90 degrees out of phase with the two portions respectively.
The invention provides a second deflection yoke having two second centering magnets mounted thereon in combination.
The plurality of positions symmetrical about a diametrical line of the annular form each serve as the most upstream position for the flow of the resin for molding the second centering magnet of the invention, so that the distribution of the content of finely divided magnetic material is symmetrical about the diametrical line. In the case where the two portions on the diametrical line are magnetized to the N pole and the S pole respectively, the distribution of the content of magnetic material is symmetrical about a line (line Bxe2x80x94B) orthogonal to the line through the center point of the N pole and the center point of the S pole. Accordingly, the distribution of the lines of magnetic force emanating from the N pole toward the S pole also becomes symmetrical about the line Bxe2x80x94B. Similarly when two portions 90 degrees out of phase with the above-mentioned two portions are magnetized to the N pole and S pole respectively, the distribution of the content of magnetic material is symmetrical about a line (line Bxe2x80x94B) orthogonal to the line through the center point of the N pole and the center point of the S pole. Accordingly, the distribution of the lines of magnetic force emanating from the N pole toward the S pole also becomes symmetrical about the line Bxe2x80x94B.
In the case of the second deflection yoke of the invention as with the first deflection yoke, the distribution of the lines of magnetic force emanating from the N pole toward the S pole in each centering magnet is symmetrical about the line Bxe2x80x94B, so that the magnetic force for the adjustment of centering can be eliminated by superposing the two centering magnets on each other as positioned in opposite relationship in polarity.
The finely divided magnetic material for use in the specific devices described contains an alnico magnetic material. The centering magnet then becomes superior to centering magnets wherein ferrite or rare-earth element magnetic material is used, in the intensity of magnetic force, temperature characteristics, cost and productivity.
The present invention provides a process for producing the centering magnet of the invention. The process has the resin molding step of molding an annular magnetic body from a magnetic resin, and the step of magnetizing the magnetic body molded by the step. Used in the resin molding step is a mold having a plurality of resin gate openings in corresponding relation with a plurality of positions symmetrical about a diametrical line of the annular form of the magnetic body to be molded. The magnetic resin is poured into the mold through the resin gate openings. The magnetizing step magnetizes the magnetic body molded by the resin molding step to an N pole and an S pole respectively at two portions thereof intersecting the diametrical line or at two portions thereof 90 degrees out of phase with the respective two portions.
According to the process described, the magnetic resin is poured into the mold first in the molding step through the resin gate openings formed in the mold, whereby the resin is divided into two streams in the vicinity of each gate opening. The two resin streams thereafter meet to form an annular magnetic body. Consequently, the magnetic body is lowest in the content of finely divided magnetic material at the most upstream position for the resin stream for molding, and is highest in the content at the confluent position. The distribution of content of the finely divided magnetic material therefore becomes symmetrical about the diametrical line.
When the two portions of the magnetic body intersecting the diametrical line are thereafter magnetized to the N pole and S pole, respectively, by the magnetizing step, the distribution of content of finely divided magnetic material becomes symmetrical about a straight line (line Bxe2x80x94B) orthogonal to a straight line through the center point of the N pole and the center point of S pole, so that the distribution of lines of magnetic force emanating from the N pole toward the S pole also becomes symmetrical about the line Bxe2x80x94B. Similarly when the magnetic body is magnetized at two portions 90 degrees out of phase with the respective two portions, the distribution of content of magnetic material becomes also symmetrical about the straight line (line Bxe2x80x94B) orthogonal to the straight line through the center point of the N pole and the center point of S pole. The distribution of lines of magnetic force emanating from the N pole toward the S pole therefore becomes also symmetrical about the line Bxe2x80x94B.
With the deflection yoke provided with a pair of centering magnets of the present invention, the adjusting magnetic force of the magnets can be eliminated completely when the centering adjustment need not be made.